High Resolution mass spectrometry will be used for the structure determination of new modified nucleosides from transfer RNA derived from prokaryotic and eukaryotic sources, and of bases and nucleosides from human urine. Microscale derivatization procedures such as trimethylsylation and O,N-permethylation will be used in these studies, followed by acquisition of mass spectra. Photographic recording will be used for field desorption and high resolution mass spectra. The fragmentation pathways of model uracil-containing nucleosides labeled with deuterium and oxygen-18 will be studied in order to gain a more complete understanding of structure-spectra relationships in the mass spectra of nucleosides.